add new tip for public S3 remotes

doc/tips/public_Amazon_S3_remote.mdwn

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+Here's how to create a Amazon [[S3 special remote|special_remotes/S3]] that
+can be read by anyone who gets a clone of your git-annex repository,
+without them needing Amazon AWS credentials.
+
+## create remote
+
+First, export your Amazon AWS credentials:
+
+	# export AWS_ACCESS_KEY_ID="08TJMT99S3511WOZEP91"
+	# export AWS_SECRET_ACCESS_KEY="s3kr1t"
+
+Now, create the remote:
+
+	# git annex initremote pubs3 type=S3 encryption=none public=yes
+	initremote pubs3 (checking bucket) (creating bucket in US) ok
+
+In the above example, no encryption was used, but it will also work
+if you enable eg, encryption=shared. It's also ok to enable chunking.
+
+The public=yes is the crtical part; this lets the public read the contents
+of the bucket.
+
+Now, copy some files to the remote, in the usual way, and push your
+git repository to someplace where someone else can access it.
+
+## use public remote
+
+Once the S3 remote is set up, anyone who can clone the git repositry
+can get files from the remote, without needing any Amazon AWS credentials.
+
+Start by checking out the git repository. 
+
+In the checkout, enable the S3 remote:
+
+	# git annex enableremote pubs3
+	enableremote pubs3 ok
+
+Now, git-annex can be used as usual to download files from that remote.
+
+----
+
+See [[special_remotes/S3]] for details about configuring S3 remotes.
+
+See [[publishing_your_files_to_the_public]] for other ways to use a public
+S3 bucket.